1. Field of the Invention
The invention relates to a receiver comprising an A/D converter for digitally sampling an analog signal modulated on a carrier frequency at a first sampling frequency, consecutively coupled to a digital quadrature mixer stage for a carrier frequency shift of the digitized modulated signal from the A/D converter, provided with first and second signal outputs via which the digital quadrature mixer stage supplies a pair of carrier converted phase quadrature signals, a digital filter device for selecting the phase quadrature signals of the quadrature mixer stage and for decimating the sampling frequency from said first sampling frequency to a second sampling frequency, and a digital demodulation device.
2. Description of the Related Art
A receiver of this type is known from European Patent Application no. 35166.
The known receiver is of the direct conversion type in which the analog RF reception signals are first digitized at the first sampling frequency and subsequently multiplied in a pair of multiplier circuits of the digital quadrature mixer stage by a pair of tunable digital phase quadrature oscillator signals which are supplied from a digital quadrature tuning oscillator. The digitized RF reception signals are thereby split in phase into I (in-phase) and Q (quadrature) signals which differ mutually 90.degree. in phase, and the digitized RF reception signal whose RF carrier frequency corresponds to the tuning frequency of the digital phase quadrature oscillator signals is also shifted in frequency towards the baseband. The desired digital I and Q baseband phase quadrature signals are selected in the digital filter device in which simulataneously the sampling frequency is reduced (decimated) from the first sampling frequency to the second sampling frequency. The digital I and Q baseband signals thus selected are subsequently demodulated in the digital demodulation device to a single digital baseband signal which is applied to a reproducing device via a D/A converter.
For an effective realization of the known receiver, the use of complex digital multipliers in the oscillator circuit as well as in the mixer stage is inevitable. An integrated embodiment therefore requires a comparatively large crystal surface. Moreover, the quadrature tuning oscillator should generate tunable digital sine and cosine oscillator signals having a comparatively low distortion. Particularly in the frequency range of normal radio and television broadcast transmitters, it is difficult to comply with this distortion requirement, which makes the known receiver less suitable for use as a receiver of broadcast signals.